1. Field of the Invention
The present invention relates to a variable displacement vane pump and, more particularly, to a structure of a pump which effectively prevents stick slip of a cam ring during its swinging movement for varying the pump displacement.
2. Description of the Prior art
Vane-pumps are known as small-sized and light-weighted pumps having a high efficiency, and have been used in various fields.
One example of variable-displacement vane-pumps out of such vane-pumps will be explained with reference to FIG. 3.
FIG. 3 is a cross sectional view of a pump portion of the variable-displacement vane pump. A drive shaft 8 is rotatably supported by a pump housing 7, and extends into a circular inner cavity of the pump housing 7 in coaxial alignment with the center axis of the inner cavity. A columnar rotor 1 is securely fixed to and rotates integrally with the drive shaft 8 in the direction shown by the arrow in FIG. 3. An annular cam ring 3 is interposed between the rotor 1 and the pump housing 7. The uppermost portion of the cam ring 3 contacts the inner surface of the pump housing 7 through a pivot member 32 while the lowermost portion of the cam ring 3 also contacts the inner wall of the pump housing 7 through a sealing member 33. Thus, the cam ring 3 swings on the pivot member 32.
The cam ring 3 is provided with a spring seat 34 at its lower portion. A coil spring 35 is disposed between the spring seat 34 and the bottom surface of a concave formed in the pump housing 7. The cam ring 3 is urged by a spring force of the coil spring 35 into its maximum eccentric position.
A plurality of vanes 2 are provided in the rotor 1 at regular intervals in a circumferential direction thereof. These vanes 2 are radially slidable inward and outward in contact with the inner surface of the cam ring 3. Upon receiving the pump discharge pressure, each vane 2 outwardly slides toward the inner surface of the cam ring 3 until a top end thereof contacts the inner surface of the cam ring 3, thereby defining closed pump chambers P together with a pair of side plates 4A, 4B, each closely facing each of both side faces of each vane 2. With the rotation of the rotor 1, each pump chamber P rotates while changing its volume.
An arc-shaped line port 41 is formed in the side plate 4B in facing relationship with the pump chamber P of which the volume gradually increases while an arc-shaped exhaust port 42 is formed in the side plate 4B in facing relationship with another pump chamber P of which the volume gradually decreases. Thus, working fluid is sucked from the intake port 41 and pressurized fluid is discharged from the exhaust port 42.
The pressurized fluid is led into a space 5a defined by a half portion of the outer periphery of the cam ring 3 ranging from the pivot member 32 to the sealing member 33, and the inner surface of the pump housing 7 by way of a regulator 92 (FIG. 5) while a space 5b defined by the remaining half portion of the outer periphery of the cam ring 3 and the inner surface of the pump housing 7 is communicated with a reservoir tank 94.
With the increase in the flow rate of the discharge fluid from the pump, and accordingly, with the increase in the discharge pressure, the cam ring 3 starts to swing leftward in FIG. 3 on the pivot member 32 against the spring force of the coil spring 35, and the center of the cam ring 3 approaches the rotational center of the rotor 1.
As the eccentricity of the cam ring 3 decreases, the volume change of the pump chambers P decreases with the result that the discharge rate decreases.
FIG. 5 shows a diagram showing the flow route of working fluid, wherein 91 designates a vane pump, and 93 designates a load. A part of pressurized fluid is led to the space 5a of the vane pump 91 through the regulator 92, which controls the pressure of the fluid led to the space 5a in response to a control signal(not shown). As a result, the pressure in the space 5a of the vane pump 91 changes in proportion to the control signal so that the displacement of the vane pump 91 is controlled in accordance with the control signal.
In the conventional vane pump having the above described construction, when a preceding vane 2 of each pump chamber P reaches the intake port 41 with the rotation of the rotor 1, and when the preceding vane 2 reaches the exhaust port 42 with the rotation of the rotor 1, the inner pressure of each pump chamber P suddenly changes. This results in eccentric loads periodically acting upon the cam ring 3 in its swinging directions, generating undesirable hunting of the cam ring 3. This hunting of the cam ring 3 causes the unstable control of the discharge rate of the pump, and causes the generation of noise.
Accordingly, conventionally, as shown in FIG. 3, the vibrations of the cam ring 3 have been prevented by providing a friction ring 6 having a rectangular cross section along one side face of the cam ring 3 over the entire length thereof. More specifically, a circular groove 36 is formed in the entire side face 3a of the cam ring 3, as shown in FIG. 4. A circular seal ring 61 is disposed within the groove 36. The friction ring 6 is brought into close contact with the side plate 4A by an elastic force of the seal ring 61. thereby generating a friction force between the friction ring 6 and the side plate 4A, and preventing the vibrations of the cam ring 3.
One example of the friction ring employed in the variable-displacement pump is disclosed in Japanese unexamined Utility Model publication No. Sho 59-160875.
However in the above-described conventional variable-displacement vane-pump, the discharge pressure from the exhaust port 42 acts upon the other side face 3b of the cam ring 3, which faces tthe exhaust port 42. This discharge pressure causes an excessively large pushing force to act upon the friction ring 6, thereby excessively increasing the friction force between the friction ring 6 and the cam ring 3, and accordingly, causing the generation of undesirable stick slip of the cam ring 3 during its swinging movement.